Vacuum tube



July 21,11936.

H. A. sNow VACUUM TUBE original Filed March 19, 1930 gwvmbob:

-15 radio receivers Patented July 2l, 1.936

VACUUM TUBE Harold n.5 snow, Mountain lakes, N. J.. assigner. by mes'ne assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware original annucaunn March 19, 1930. serial No.l

Divided and this application March '12, 1931, Serial No. 522,062. Renewed January 18, 1936. In Canada September 13, 1930 7 Claims.

This invention relates to vacuum or electronic tubes suitable for the amplificationof modulated carrier waves.

This application is a division of -my copending application, Ser. No. 437,225, filed March 19, 1930 and, for a more extended discussion of the method of operation and the advantages of vacuum tubes'of the poly-mu, or variable mu-factor,

type disclosed and claimed in this application,`

reference is made to the aforesaid application and to the article by Stuart Ballantine and Harold A. Snow in the Proceedings of the Institute of Radio Engineers, December, 1930, pages 2102 to 2127, Reduction of distortion and cross-talk in by means of variable-mu tetrodes".

An object of the invention is to provide a vacuum .tube in which the electrodes are constructed and arranged to produce a desired relation betweenl the current flow in one electrode circuit and the voltage on another electrode, for example, between plate current and control grid voltage whereby practically no distortion is introduced when, forv increasing signal strengths,

the operating potentials are so adjusted that the amplification rate is reduced to a small fraction of the maximum amplification, and the tube will give` an undistorted output of approximately constant magnitude over a wide range of applied "so carrier voltages. A further object is to provide a tube of the ty'pe stated in which the control grid 'is so constructed that the space current path.

adjacent different parts of the cathode is characterized by substantially different mu factors 85 and transconductance values so that when the potentials are adjusted to give a relatively low amplification of strong signals, the change in transconductance for a given change in the gain control voltage is much lower than is the case 40 with the known types of tubes. A further object is to provide a vacuum tube of the type stated having an equipotential cathode and in which the control grid has the'form of a helical winding of non-uniform pitch. More specifica1ly, an ob- 45 ject of the invention is to provide a vacuum tube of the type stated having coaxial cylindrical elements and in which the control grid has the form of a winding made up of a plurality of sections of helical windings, each section being of sub- 50'stantially uniform pitch but of different pitch thanthe adjoining section.

'Ihese and other objects of the invention will be apparent from the following specification when taken with the accompanying drawing,.in

55 which Fig. l is a perspective view of a vacuum tube embodying the invention, partsr being broken away to illustrate the construction of the control grid,

Fig. 2 is an enlarged detail view of a control 5 grid such as shown in Fig. 1,

Fig. 3 is a, diagrammatic view illustrating another embodiment of the invention.

Figure 4 is a curve sheet showing the variations of plate current with grid bias fora tube 10 such as shown in Figure 1.

Figure 5 is a curve sheet showing the relation between control grid voltage and transconductance for tubes embodying the invention.

In Fig. 1, the invention is illustrated as em- 15 bodied in a form of vacuum tubeknown commercially asV a screen grid tube having a separate heater for energizing an equal-potential cathode. As is well known, this particular type of tube comprises an evacuated envelope I enclosing a 20 cathode 2 heated by a 'resistance (not shown) within the cathode, an inner or control Vgrid G1.

a screen grid G2, a plate P and an outer screen S. Except for the novel construction of the control grid G1 and its novel functional relationship to 25 the remaining elements of the tube, the several elements of the tube and their physical arrangement may be substantially the same as that employed in the present commercial tubes.

In tubes of this general type, the control grid Gi comprises a helical winding, of uniform pitch, supported by one or more wires which extend parallel to the axis of the cathode 2.

In accordance with the present invention, the desired operating characteristics of the tube, as explained in my copending application and the aforesaid publication, are obtained by making the control grid winding of non-uniform pitch.

As shown in Fig. 2, the control grid G1 takes the form of a continuous helical winding 3 with 40 its turns constituting transverse conductors connected to and supported by one or more foundation supports or wires 4, and having a. pitch 'such that each turn of the helical winding is spaced from the adjacent turns a distance greater than the diameter of the wire and sufdcient to permit n the space current or electron lstream from the cathode to the anode to pass through and to be controlled by substantially all of the winding of the grid. In the particular form of control grid shown, the end sections a of the winding have the same relatively close pitch and the central section b has a longer pitch, i. e., a greater spacing between turns of the winding than the. adjoining end section a. v

The invention may be employed with triodes, as illustrated in Fig. 3, and various arrangements of -a plurality of sections of different pitch may be employed in tubes of the triode and tetrode or As shown in Fig. 3,.

By transconductance is meant the ratio of the change in the current in the circuit of an relectrode to the change in the voltage on another electrode, under the condition that all other voltages remain unchanged. By mu-factor is meant the 'ratio of the change in one Aelectrode voltage to a change in the other electrode voltage, under the condition that a specied current remains unchanged..

As explained in more detail in my copending application of which this application is a division, the amplication of a vacuum tube may be regu-v lated by adjusting the bias voltage upon the control grid, the amplification decreasing as the bias voltage becomes more negative. Curves showing the relation between plate current and grid bias,

i. e., transfer characteristics, afford an indication ofthe amplification at different bias voltages, since the slope ofthe curve at any point is a measure of the amplication when the tube is biased for voperation at that'point.

' In Figure 4, the solid line curve A is the transfer characteristic for a tube such as shown in Figure 1, and the dotted line curve B is a similar curve for a. commercial screen grid tube of the same general type, but having a' continuous control grid winding of uniform pitch. A11-examination of curve A shows that, with tubes embodying the invention, a control of amplication extends over a range of control grid bias of from zero to more than -30 volts. With known tubes, the curvature of the transfer characteristic approaches zero at a control grid bias of about -15 volts.

In other words, an increase in the grid bias above approximately 15 volts negative will not be accompanied by a decrease in amplication when the known type of tube construction is employed, but with tubes embodying the invention, the amplification may be varied with changes of control grid bias throughout a range of from zero to upwardly of 30 volts. The tubes will still passsignals, by leakage transmission whenthe control grid biases exceed these respective values. but control of amplification is no longer possible in regions where the transconductance curves become substantially horizontal.

The curves of Figure 5 show the relation bel tween control grid bias and transconductance for two tubes embodyingl the invention, and fora similar tube which has the usual grid construction.- Curve A is the transconductance-,control grid biascurve for a screen grid tube of the type shown invFigure 1, having two turns omitted from the center of the control grid. 'I'he data for this curve and for curve A of Figure 4 relates to the same tube. Curve A' is a similar curve for a screen grid tube in which only one turn was removed from the center of the control grid, and curve B` shows the characteristic properties of the conventionall type of tube having a continuous control grid winding.

g An examination of these curves shows that the useful range of transmission control is"consid erably extended by -the present invention. With amazes conductance from about 500 micromhos to lthe value, about 0.8 micromho, at which leakage transmission prevents further amplification control,'corresponds to a change in 'control grid bias of about ten volts. The corresponding ranges of control grid bias for the tubes of curves 'A' and A are, respectively, about 30 and 60 volts.

As stated above, modulation distortion may occur when, for a given signal strength, the ampli-v cation is so adjusted as to bring the voutput down to a desired, or standard, level. Since such distortion is due to the curvature of the transfer characteristic, it will be apparent that a tube having a curve of lower curvature can transmit, without distortion, higher voltage signals than a tube having a characteristic which exhibits a region of higher curvature. An examination of the curves of Figure 4, will show vthat the maxi,

mum curvature of curve A is substantially lower 2 -38 volts. For thetube with two turns removed f from the control grid, curve'A shows a permissible input voltage of 3 volts, with a bias of ,-95 volts.

. Furthermore, by so restricting the control grid bias voltages that the maximum can never ex- 30 ceed about 28 volts negative in the case'of the tube of curve A', and about 65 volts in the case of the tube of curve A, the maximum carrier voltages which may be transmitted are approximately '7 and 22 volts, respectively.

Observations of actual performance in a receiver check closely with the resultsl plotted in Figure 5 for single stages. By employing tubes Aconstructed in accordance with the invention, the

permissible input voltage was raised from 0.3 volt to 17 volts, i. e., volume control with good quality reproduction can be had with input volt-v1 i ages about 57 times as great as those which may be applied when the` known commercial form of tube is employed.

It will be apparent to those skilled in the art that my invention provides means for extending the range of signal voltages which may be transmitted without distortion, the signals being either of audio orv radio frequency, and if of radio frequency, either continuous wave or modulated.

I claim:

1. A control grid adapted ior use in a variable mu-factor tube having an equi-potential cathode, anode and screen grid, and comprising-a helical winding having a pair of .sections of different pitch with the turns of each section spaced to permit an electron stream to ilow through and be controlled by said section, the pitch throughout each of the sections being uniform.

2. A control gridv adapted for use in a variable mu-factor tube havingan equi-potential cathode, anode and screen grid, and comprising a helical winding including two sections of the same uniform and suiilciently great pitch to permit an electron stream to ow through and be controlled by each of said sections, and an intermediate section of a uniform wider pitch proportioned to control an electron stream through said intervmediate section.

3. An electron discharge device having as elements a cathode, an anode, and a control grid interposed between said cathode and said anode,

said grid comprising foundation supports and spaced transverse conductors connecting said spacing, said sections being mounted end to end and each section having all its conductors spaced apart.

5. An electron discharge device comprising an equipotential cathode, a cylindrical anode surrounding and coaxial with said cathode, and a.

cylindrical control grid interposed between and coaxial lwith said cathode and said anode and comprising a wire helically' wound into adioining sections each of uniform pitch and with all its turns spaced apart, said sections being mounted end to end to form said grid with one section in registry with one longitudinal portion of 'said cathode and another section having a pitch greater than the average pitch of said grid in registry with another longitudinal portion of said cathode.

6. A thermionic electron discharge tube comprising a cathode, an anode, and an interposed control grid comprising foundation supports and said grid having a plurality of sections adjoined end to end, each section being of uniform spacing and having its conductors spaced to control vtransverse conductors connecting said supports,

the electron current from said cathode to said Y anode, one of said sections extending along one portion of the length of said cathode and another of said sections having its conductors spaced a greater distance than the other section extending along another portion of the length of said cathode.

7. An electron discharge device comprising a cathode, a control grid surrounding said cathode, a screen surrounding said control grid, and an anode surrounding said screen, said control grid comprising foundationsupports with transverse conductors connected to said supports and spaced 20 to control the electron current from said cathode to said plate and having a plurality oi sections each of uniform pitch and adjoined end to end with one of said sections covering one part`of ing greater than the average spacing of said grid covering another part ot said cathode.

HAROLD A. SNOW.

lsaid cathode and another sectionhaving a spac- 25 

